(1) Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary cell having a construction wherein a flat spiral-shaped electrode assembly, in which positive and negative electrodes capable of intercalating and deintercalating lithium ions are wound with a separator disposed therebetween, is enclosed in a casing which changes shape with a slight increase in the internal pressure of the cell, and a gel polymer containing a nonaqueous liquid electrolyte exists between the positive electrode and the separator, bonding the positive electrode and the separator. The present invention also relates to a method of producing the same.
(2) Description of the Prior Art
Conventionally, only materials composed of metals such as stainless steel have been used for the casings of nonaqueous electrolyte cells. However, in cells that utilize this kind of casing, the casing made of metal must be thick, and along with this the cell mass increases. As a result, difficulty in reducing the thickness of the cell and at the same time, a decline in the mass energy density of the cell, were problems.
In an attempt to overcome these problems, the inventors of the present invention made an aluminum laminated film, being composed of resin layers formed on both surfaces of a metal layer comprising aluminum with adhesive layers disposed therebetween, into a pouch to construct a laminated casing, and proposed a thin cell having an electrode assembly enclosed in the enclosure space of this laminated casing. With a cell having this kind of construction, there are the advantages of remarkable reduction in the thickness of the cell and furthermore of an increase in the mass energy density of the cell.
However, because the casing of a cell utilizing the above-described laminated casing is more flexible than that of a cell utilizing a metal casing, the cell utilizing the laminated casing has the following disadvantages when overcharged. When the cell is overcharged to about 200% of cell capacity, the liquid electrolyte and the gel polymer begin to oxidize and decompose at the positive electrode, gas is generated, and consequently the temperature of the cell begins to rise. Due to the generating of gas, the bonded portion of the positive electrode and the separator begins to detach, overvoltage arises (the effective area of the electrode decreases), and the charge rate per unit area increases, resulting in partial shutdown of the separator. In this kind of state, because, when charging is continued, the effective area of the electrode further decreases inviting more current concentration on the electrode plates, an abnormal amount of heat is generated in portions of the electrode plates. Consequently, because the separator melts and a short circuit results, a decline in the safety of the cell becomes a problem.
Therefore, it is an object of the present invention to provide a nonaqueous electrolyte secondary cell intended to improve safety through prevention of a short circuit in the cell caused by detachment of the bonded portion of the positive electrode and the separator, even when the cell is overcharged and gas is generated.
It is another object of the present invention to provide a method of producing a nonaqueous electrolyte secondary cell in which the nonaqueous electrolyte secondary cell intended to improve safety can be easily prepared.
These and other objects are accomplished in one aspect of the invention by providing a nonaqueous electrolyte secondary cell comprising:
a flat spiral-shaped electrode assembly comprising a positive electrode, a negative electrode, and a separator disposed therebetween, the positive electrode and negative electrode being capable of intercalating and deintercalating lithium ions, and the electrode assembly being enclosed in a casing which changes shape with a slight increase in the internal pressure of the cell; and
a gel polymer containing a nonaqueous liquid electrolyte, the gel polymer existing between the positive electrode and the separator and bonding the positive electrode and the separator;
wherein the adhesive strength between the separator and the positive electrode is 0.02 N/10 mm or higher.
The overcharge level (the level of current at which problems do not arise during overcharging) in a cell, as shown in the Formula (1) below, is proportional to stack strength (the adhesive strength between the positive electrode and the separator) and is inversely proportional to the amount of gas generated. Therefore, if the adhesive strength between the positive electrode and the separator is 0.02 N/10 mm (2 gf/10 mm) or higher as in the above construction, even when the cell is overcharged, oxidation and decomposition of the liquid electrolyte and the gel begins at the positive electrode, and gas is generated, because the stack strength is large, detachment of the bonded portion of the positive electrode and the separator is suppressed. Thus, because it is possible to ensure prevention of a shutdown of the separator caused by a reduction in the effective area of the electrode, or a short circuit in the cell caused by the melting of the separator, the overcharge level increases.                                                                         Overcharging level                            ∝                              xe2x80x83                            ⁢                                                (stack strength)                                                  (amount of gas generated)                                                                                                        ∝                              xe2x80x83                            ⁢                                                                    (surface area of separator)                                    *                                      
                                    ⁢                                      xe2x80x83                                    ⁢                                      (strength of gel polymer)                                                                    1                  /                                      (oxidation potential of gel polymer)                                                                                                                          ∝                              xe2x80x83                            ⁢                                                                    xe2x80x83                                    ⁢                                                            [                                              1                        /                                                  (porosity of separator)                                                                    ]                                        *                                          
                                        ⁢                                          (proportion of polymer in gel)                                                                                        1                  /                                      (oxidation potential of gel polymer)                                                                                                          (        1        )            
In another aspect of the invention, the casing is a laminated casing.
In another aspect of the invention, the porosity of the separator is 60% or less and the proportion of polymer component in the gel polymer is 5 mass % or more.
As shown in the above Formula 1, the stack strength is inversely proportional to the porosity of the separator, and proportional to the proportion of the polymer component in the gel polymer (in Formula 1, this is abbreviated as proportion of polymer in gel). Through experiments carried out by the inventors of the present invention, it was found that when the porosity of the separator is 60% or less, and the proportion of polymer component in the gel polymer is 5 mass % or more, the adhesive strength between the positive electrode and the separator is 0.02 N/10 mm or higher. Therefore, it is preferable that the porosity of the separator and the proportion of the polymer component in the gel polymer be fixed as in the above aspect of the invention.
In another aspect of the invention, the porosity of the separator is 45% or higher and the proportion of polymer component in the gel polymer is less than 30 mass %.
The porosity of the separator and the proportion of polymer component in the gel polymer are fixed in this way because when the porosity of the separator is less than 45% and the proportion of polymer component in the gel polymer is 30 mass % or higher, although the adhesive strength between the positive electrode and the separator becomes very strong, cell characteristics such as the discharge characteristic deteriorate. Therefore, it is preferable that the porosity of the separator and the proportion of the polymer component in the gel polyester be fixed as in the above aspect of the invention.
In another aspect of the invention, the oxidation potential of the gel polymer is 4.8 V or higher versus Li/Li+.
As shown in the Formula 1, the amount of gas generated is inversely proportional to the oxidation potential of the gel polymer. Through experiments carried out by inventors of the present invention, it was found that when the oxidation potential of the gel polymer is 4.8 V or higher versus Li/Li+, it is possible to sufficiently control the amount of gas generated. Therefore, it is preferable that the oxidation potential of the gel polymer be fixed as in the above aspect of the invention.
In addition, the objects of the invention are accomplished, in another aspect of the invention, by providing a method of producing a nonaqueous electrolyte secondary cell comprising the steps of:
preparing an electrode assembly having a flat spiral-shape by winding a positive electrode and a negative electrode with a separator disposed therebetween, the positive electrode and the negative electrode being capable of intercalating and deintercalating lithium ions;
enclosing the electrode assembly in a casing which changes shape with a slight increase in the internal pressure of the cell; and
putting together the positive electrode, separator, and the negative electrode such that after a pregel comprising a liquid electrolyte and a polymer precursor is poured into the casing, the pregel crosslinks and polymerizes by heating to form a gel, and the adhesive strength between the positive electrode and the separator is 0.02 N/10 mm or higher.
With the above-described method of production, a nonaqueous electrolyte secondary cell according to the present invention can be easily prepared.